It has been known that there is a vehicle driving assist apparatus that recognizes a driving zone from left and right lane markings arranged along a road on which a vehicle is running and controls at least one of the steering and braking of the vehicle so that the vehicle may run without deviating from the driving zone (refer to Japanese Unexamined Patent Application Publication No. 2009-214786). If a space between the lane markings widens to an extent improper to define the driving zone, Japanese Unexamined Patent Application Publication No. 2009-214786 sets, as a reference, one of the left and right lane markings that extends along a planned driving road of the vehicle and recognizes a virtual driving zone (driving zone).
Lane keep control is used not only for keeping a driving zone but also, in some cases, for maintaining a prescribed position, for example, a central position with respect to lane markings of a lane. In such a case, a change in a space between the lane markings results in changing the prescribed position with respect to the lane markings of the lane. If a space between lane markings changes and if the lane keep control is carried out with a regular control amount, a lateral position of the subject vehicle will be destabilized to give a driver an odd feeling.